Energy-dispersive radiation spectrometry systems, such as, without limitation, X-ray spectrometry systems or gamma-ray spectrometry systems, are used for detecting, measuring and analyzing radiation emissions, such as X-ray emissions or gamma-ray emissions, from, for example, a scanning electron microscope (SEM). A typical energy-dispersive radiation spectrometry system includes the following four main components: (1) a detector, (2) a pre-amplifier, (3) a pulse processor, and (4) a computer-based analyzer. For convenience only, and not for purposes of limitation, the following description will relate to X-ray spectrometry systems and photons in the form of X-rays (as compared to, for example, photons in the form of gamma-rays that are detected in a gamma-ray spectrometry system).
The detector, which usually takes the form of a semiconductor sensor of some type, converts an incoming X-ray into a very small current pulse, typically on the order of tens of thousands of electrons, with a duration of about tens to a few hundreds of nanoseconds. The magnitude of each of the current pulses is proportional to the energy of the X-ray.
The pre-amplifier amplifies the current pulse output by the detector and typically converts it into a voltage signal in the range of tenths of millivolts up to a few hundreds of millivolts. There are two main types of preamplifiers: “tail pulse” or RC-coupled preamplifiers, and pulsed-reset preamplifiers. The subject matter described elsewhere herein applies to both types of preamplifiers.
In a pulsed-reset type of preamplifier, the charge generated in the sensor is integrated in a feedback capacitor such that the resulting voltage increases in steps of varying heights and intervals, until it reaches an upper limit. When that limit is reached, a “reset” pulse is applied which drains the accumulated charge from the feedback capacitor, restoring the preamplifier to near its minimum output voltage in a short time, typically a few microseconds. Then, charge due to the interaction of X-rays with the detector accumulates on the feedback capacitor again, and the cycle repeats. In contrast, tail-pulse preamplifiers act as high-pass filters on the voltage step signal output by the detector, with an exponential return to baseline whose time constant is long compared to the charge integration time in a feedback capacitor of the preamplifier.
The pulse processor receives the pre-amplifier signal and generates a numeric representation of the X-ray's energy through an integration process. In older energy-dispersive radiation spectrometry systems, the pulse processor included two separate components, namely a “shaping amplifier” and an analog to digital converter. Modern energy-dispersive radiation spectrometry systems, on the other hand, typically combine these functions, with the newest designs digitizing the preamplifier signal directly and carrying out all pulse detection and filtering functions using digital signal processing.
The computer-based analyzer accumulates the X-ray energies output by the pulse processor into a spectrum or plot of the number of X-rays detected against their energies. The spectrum is divided into a somewhat arbitrary number of small ranges called “channels” or “bins.” In older systems, a hardware component called a multi-channel analyzer (MCA) did the accumulation of X-rays into spectrum channels and a computer read out the summed result. In modern systems, the MCA function is handled in software, either by the computer or even within the pulse processor.
The job of the pulse processor is made more complex by several factors. For example, electronic noise is superimposed on the underlying signal received from the preamplifier. For X-rays that are near the lowest detectable energy level, the preamplifier output step height may be significantly smaller than the peak-to-peak excursions of the electronic noise. In such as case, the X-ray can only be detected by filtering the signal for a relatively long period of time before and after the step, to average away the contribution of the noise. The amount of such noise averaging is a fundamental operating parameter of all pulse processors. This averaging time is variously referred to in the art as “shaping time” or “peaking time.”
Second, the steps in the preamplifier output are not instantaneous. In the absence of noise, the signal would be a sigmoidal (S-shaped) curve. This is due to bandwidth limitations, device capacitance, and the time required for all the electrons generated by an X-ray to reach the anode of the sensor. These electrons can be visualized as a small cluster or cloud, which moves through the sensor material toward the anode under the influence of the bias voltage field within the semiconductor sensor. With a tail-pulse preamplifier, the initial rise of the signal is of the same sigmoidal form, followed by an exponential decay whose time constant may vary with the design but is always long compared to the initial rise.
In a traditional detector with simple planar electrodes on each face, called a lithium-drifted silicon or Si(Li) detector, the bias field lines are straight (to a first approximation, ignoring edge effects) and run front-to-back. As a result, the electron cloud collection time is approximately constant, and the “rise time” of the preamplifier signal (the width of the sigmoidal step) is dominated by bandwidth limitations due to the relatively large capacitance of the device.
A new type of sensor has been developed in recent years, known as a Silicon Drift Detector (SDD). Its salient novel characteristic is a concentric pattern etched into the bias electrodes which, when slightly varying voltages are applied to the individual rings in the pattern, permit the bias field inside the sensor material to be shaped such that the electrons are funneled to a very small spot anode. This has the effect of reducing the effective device capacitance by about four orders of magnitude. The electron cloud from an X-ray interaction expands with drift time to a greater or lesser degree depending on the path length it travels to reach the anode. Because of the reduced device capacitance, the cloud integration time contributes much more to the rise time of the preamplifier signal, which in SDDs can vary by about a factor of two, as compared to a few percent in the case of Si(Li) detectors (although even the longer end of the range of the rise time for an SDD may still be faster than a conventional planar-electrode sensor (Si(Li) detector) due to the reduced total capacitance).
A phenomenon known in the art as “pulse pile-up” occurs as a result of successive X-rays arriving too close together to have their energies measured independently. If undetected, only one energy is measured for both X-rays, located somewhere between that of the higher-energy X-ray of the pair and the sum of the two X-ray energies, depending on the details of the pulse shaping filters of the system and the time interval between the X-rays. Thus, pulse processors need to be able to effectively detect the occurrence of pile up, which when detected, will result in the energy measurements associated therewith being discarded (referred to as pile up rejection).
Radiation, whether naturally occurring or induced by some form of excitation, is a random process. No matter how high or low the average emission rate, with some non-zero probability the time interval between two emitted X-rays can be arbitrarily short. The probability of getting a second X-ray within any time interval t is:P=(1−e−(rt))
where e is the base of natural logarithms and r is the average X-ray arrival rate.
The minimum time interval between two X-rays which still permits them be identified as distinct events, which is known in the art as the “pulse-pair resolving time”, is a strong inverse function of energy. In other words, it is much more difficult to detect near coincidences between small (low energy) pulses than large ones. Because all peak-detecting filters of a pulse processor respond strongly to high-energy X-rays, the most difficult case to detect is a closely following low-energy X-ray.
The traditional method of pile-up detection can generally be described as one or more parallel filters with fixed but very short shaping times compared to the shaping time of the main energy-measurement processing path (referred to as the “main channel”). These are variously called “fast channels” or “pile-up rejection channels”. Each channel (main and fast) has a parameter referred to as dead time, which is the amount of time it takes the channel to accurately and unambiguously measure the energy of a single X-ray. Because the fast-channel dead times Df will be much shorter than the dead time D of the main channel, the fast channels are much more likely to produce distinct pulses for X-rays arriving close together in time. The filters (analog or digital) which are used in the fast channels are generally of the same type used for energy measurement (the main channel), just with much shorter pulse widths.
However, because the fast-channel shaping times are so short, they are not very effective at averaging away electronic noise. The shaping time of any pulse processing channel determines the lowest energy X-ray which can be detected in that channel. If its detection threshold is set any lower, the processing channel will produce excessive false triggers due to the random noise fluctuations in the preamplifier output signal. A state of the art X-ray spectrometry system will typically be able to distinguish X-rays of about 100-200 electron volts (eV) from noise in the main measurement channel, but the threshold energies of the fast channels must be much higher. The fastest pile-up rejection channel, which defines the best pulse-pair resolving time for high-energy X-rays, typically has a threshold between 1000-2000 eV. Some existing pulse processors have as many as three pile-up rejection channels to improve pile-up rejection performance in the range below 1000 eV. In systems with more than one pile-up rejection channel, the intermediate channels will have shaping times chosen to allow sensitivity to particular emission lines such as Oxygen at 525 eV, or Carbon at 277 eV. With each step down in the desired energy detection threshold, the pulse-pair resolving time is degraded by the need for longer shaping time.
The pulse-pair resolving time is dominated by the lower-energy X-ray of the pair. This is important because low-energy pile-up detection failure affects not only low-energy peaks, but all peaks in the spectrum. Undetected pile-up with low-energy X-rays can shift counts out of any peak into a broad shelf extending from its expected position as far as the sum of the two peak energies. A good description of the dependence of pile-up effects on energy may be found in P. J. Statham, Microchim. Acta 155, 289-294 (2006).
Furthermore, the highly variable rise time for valid single X-ray pulses in the case of SDDs, depending on how far from the charge-collection anode the X-ray is absorbed, poses the biggest challenge for traditional methods of detecting very close coincidence in time, when even the fastest conventional pile-up channel produces only a single output pulse. The classic technique, as described in, for example, U.S. Pat. No. 5,684,850 to Warburton et al., is a pulse-width test. Digital triangle or trapezoidal filters are most popular for all-digital pulse processing systems, because they are relatively easy to construct and computationally efficient. There is also what is known in the art as Finite Impulse Response (FIR) filters, meaning the response of the filter is guaranteed to be zero outside a finite range of time defined by extent of the filter's non-zero weighting coefficients. In contrast, traditional semi-Gaussian analog pulse shaping introduces exponential time constants whose response in principle continues forever, although in practice the output decays below the noise threshold in a reasonably predictable (albeit slightly energy-dependent) time.
The pulse width of FIR filters is not energy-dependent even in principle, but it is dependent on the rise time of the preamplifier step, which is in turn dependent on the variable charge collection time in the SDD. Thus, in order to avoid false rejection of valid pulses from single X-rays, a fixed pulse-width test must be set long enough to accept the maximum rise time resulting from the longest drift path length in the SDD.
It would thus be advantageous to have a pile up detection method that is not dependent on rise time, as such a method would improve the performance of systems employing SDDs wherein rise times are highly variable.